Many methods are known for forming a patterned conductor layer on a substrate. The two most common methods of forming such a layer are subtractive etching and lift-off techniques. In subtractive etching, after a blanket conductor layer is deposited on the substrate, the layer is etched through a photomask in order to remove undesired portions thereof. In lift-off, a layer (typically an insulator such as polyimide) is deposited on a substrate, and is patterned through a photomask. The conductive layer is then deposited on the patterned insulator, and the insulator is removed from (i.e. "lifted off" of) the substrate, taking with it the undesired portions of the conductive layer. Of these two techniques, it has been found that lift-off is more desireable in that the solvents used to remove the insulator in lift-off cause less damage to the underlaying substrate than do the etch processes (e.g. a plasma etch or a reactive ion etch) used in subtractive etching. Also, the conductor profile resulting from lift-off processing minimizes step coverage problems in subsequent conductor layers.
An example of such a lift-off process is disclosed in U.S. Pat. No. 4,451,971, entitled "Lift-Off Wafer Processing", issued June 5, 1984 to Milgram and assigned to Fairchild Camera and Instrument Corp. As disclosed in this patent, a layer of pre-imidized polyimide (i.e. a copolymer of an aromatic cycloaliphatic diamine and a dianhydride) is coated on a semiconductor substrate, and a silicon dioxide barrier layer is formed on the polyimide. The barrier layer protects the polyimide layer during photolithographic processing. After these layers are patterned through a photoresist mask, a metal layer is deposited on the structure. The polyimide layer is then stripped off the silicon, lifting off the undesired portions of the metal layer. By use of the particular polyimide copolymer disclosed, the metal can be deposited at a temperature of 250.degree. C.-300.degree. C., reducing physical faults in the deposited metal. Note that during both the deposition and lift-off of this polyimide copolymer, a harmful organic solvent such as methylene chloride must be used.
In the article by Homma et al, "Polyimide Liftoff Technology for High-Density LSI Metallization", IEEE Transactions on Electron Devices, Vol. ED-28, No. 5, May 1981 pp. 552-556, a lift-off metallization process is disclosed in which a polyimide having a high imidization temperature, sold under the trade name "PIQ" by the Hitachi Chemical Co., Ltd of Japan, has an overlaying molybdenum barrier layer formed thereon. The PIQ serves as the lift-off structure (i.e. the layer which is lifted off from the underlaying layers).
In the article by Winter, "Metal Deposition With Polyimide Lift-Off Technique", IBM Technical Disclosure Bulletin, Vol. 17, No. 5, Oct. 1974, p. 1309, a first layer of polyimide is patterned through a photoresist mask. After the metal is deposited, the photoresist mask is removed from the first polyimide layer and a second polyimide layer is applied for passivation.
As discussed above, special polyimide layers are needed in order to carry out high temperature lift-off processes. However, these special polyimides are typically used in conjunction with overlaying barrier layers, which protect the polyimides from etching during the definition of a photoresist mask disposed on the barrier layer. It would be advantageous to eliminate these barrier layers, since they add to manufacturing cost.